1. Field of the Present Invention
The present invention relates to a planographic printing plate precursor capable of being exposed by an IR laser for image formation thereon. More specifically, the present invention relates to such planographic printing plate precursor having a negative recording layer of high recording sensitivity.
2. Description of the Related Art
The recent development of laser technology has been remarkable, and high-power, small-sized solid lasers and semiconductor lasers for emitting near-IR and IR rays have become readily available. For light sources for directly processing printing plate precursors from digital data of computers or the like, these lasers are extremely useful.
Negative planographic printing plate materials for IR lasers, that is, materials to be processed for image formation thereon, with an IR laser capable of emitting IR rays as a light source, generally have a photosensitive layer that comprises an IR absorbent, a polymerization initiator capable of generating a radical when exposed to light or heat, and a polymerizable compound.
One example of such negative image recording materials is described in U.S. Pat. No. 5,340,699, which features an IR absorbent, an acid generator, a resol resin and a novolak resin. However, negative image recording materials of this type require heat treatment at 140 to 200xc2x0 C. for 50 to 120 seconds or so, after exposure to a laser for image formation thereon, and this heat treatment often requires a large, complicated device and much energy.
Japanese Patent Application Publication (JP-B) No. 7-103171 discloses a recording material that includes a cyanine dye having a specific structure, an iodonium salt, and an ethylenically unsaturated double bond-having addition-polymerizable compound. This does not require heat treatment after imagewise exposure to light. However, the recording material disclosed is problematic in that the polymerization of the polymerizable compound therein is often retarded by oxygen in air, and therefore sensitivity is not satisfactory. Japanese Patent Application Laid-Open (JP-A) No. 8-108621 discloses an image-recording medium that features an ordinary thermal polymerization initiator, which is an organic peroxide or azobisisobutyronitrile compound, and a thermo-polymerizable resin. Regarding image-recording sensitivity, however, this medium requires an energy level of at least 200 mJ/cm2. Accordingly, to increase sensitivity, the medium must be pre-heated before exposure to light. At present, no one has succeeded in realizing high-sensitivity recording materials satisfactory for practical use.
An object of the present invention is to provide a negative planographic printing plate precursor of high sensitivity, which can be imagewise exposed by IR rays from an IR-emitting solid laser or semiconductor laser for direct image formation thereon from digital data of a computer or the like, without requiring a heat treatment after this exposure to light for image formation.
Having specifically noted the constituent components of negative image-recording materials and having assiduously studied them, the present inventors have found that, when an onium salt whose counter anion has a divalent anionic structure is used for a polymerization initiator, the recording sensitivity of an image-recording material can be increased. On the basis of this finding, we have completed the present invention.
Specifically, the present invention provides a negative planographic printing plate precursor for a heat-mode exposure system, the plate precursor having, on a support, a photosensitive layer that is exposable with an IR laser, the photosensitive layer including: (A) a light-to-heat conversion agent; (B) a polymerizable unsaturated group-having compound; and (C) a polyvalent anionic onium salt having a counter anion that has a valency of at least two.
Although not clear, the mechanism of the planographic printing plate precursor of the present invention is thought to be as follows: In the plate precursor, the counter anion of the onium salt that serves as an initiator, such as a sulfonium, iodonium, diazonium or azinium salt, has a divalent anionic structure. Therefore, the electron density of the counter anion is high, and thermal decomposition of the onium salt is thereby facilitated. In addition, an ordinary light-to-heat conversion agent such as an electrically-charged cyanine dye or oxonole dye can readily interact with an onium salt of this type, and therefore the dye and the initiator are readily localized to thereby increase light-to-heat conversion efficiency of the plate precursor. Accordingly, the initiator can be efficiently decomposed, increasing the recording sensitivity of the plate precursor.
The planographic printing plate precursor of the present invention may be for a xe2x80x9cheat-mode exposure systemxe2x80x9d, which means that the plate precursor may be subjected to heat-mode exposure for image formation thereon. A definition of heat-mode exposure is now described in detail. As described by Hans-Joachim Timpe (IS and Ts NIP 15:1999 International Conference on Digital Printing Technologies, page 209), it is known that a process featuring photo-excitation of a light-absorbing substance (e.g., dye) in a photographic material followed by a chemical or physical change thereof for image formation in a photosensitive layer of the material (that is, a process of image formation comprising photo-excitation of the light-absorbing substance followed by the chemical or physical change thereof includes two major modes. Specifically, one is a photon mode in which the photo-excited light-absorbing substance in the photographic material is inactivated through some photo-chemical interaction (for example, energy transfer or electron transfer) with another reactive substance in the material, and the reactive substance, having been thus activated as a result of the interaction, undergoes the chemical or physical change necessary for image formation in the photosensitive layer of the material. The other mode is a heat mode in which the photo-excited light-absorbing substance in the photographic material generates heat and is thus inactivated by the heat generation, and the other reactive substance in the material receives the heat and undergoes the chemical or physical change necessary for image formation in the photosensitive layer of the material. Other minor modes of the process, for example, ablation, in which the substances in a photographic material are explosively scattered by locally focused light energy, and poly-photon absorption, in which one molecule in a photographic material absorbs a number of photons at the same time, are omitted herein.
The exposure processes of the modes are referred to as photon-mode exposure and heat-mode exposure. A technical difference between photon-mode exposure and heat-mode exposure is whether or not the energy quantities from a plurality of photons for exposure can be added up for the intended reaction. For example, referred to is a reaction through exposure to a number of photons n. In photon-mode exposure, which takes advantage of photo-chemical interaction of the substances in the photographic material, the energy quantities from n photons cannot be added up for the reaction, because of the laws of quantum energy and momentum conservation. In other words, every reaction through photon-mode exposure requires the condition xe2x80x9cquantity of energy of one photonxe2x89xa7quantity of energy for one reactionxe2x80x9d. On the other hand, in heat-mode exposure, the light-absorbing substance in the photographic material is first photo-excited to generate heat, and the heat, having been thus converted from light energy, serves for the reaction for image formation in the photosensitive layer of the material. Accordingly, in heat-mode exposure, the energy quantities of all n photons can be added up for image formation. Therefore, the condition xe2x80x9cenergy quantities of n photonsxe2x89xa7energy quantity for one reactionxe2x80x9d is sufficient for heat-mode exposure. However, the addition of the energy quantities in heat-mode exposure is restricted by heat diffusion. Concretely, when an exposed area (reaction point) of a photographic material successively undergoes a subsequent photo-excitation and inactivation before heat generated by a previous photo-excitation and inactivation step is dispersed by heat diffusion, and therefore that area successively receives heat through subsequent photo-excitations and inactivations, then the heat quantities can be surely accumulated and added up to thereby elevate the temperature of the exposed area. However, when the heat generation in the next step is delayed, the heat generated in the previous step will disperse from the area through heat diffusion. In other words, in heat-mode exposure to a predetermined level of total energy, a case of short-time exposure to higher energy and a case of long-time exposure to lower energy produce different results, and the former case of short-time exposure to higher energy is more advantageous than the latter case.
Photon-mode exposure may also undergo this same phenomenon, of being influenced by subsequent reactions, but is basically free therefrom.
The difference between photon-mode exposure and heat-mode exposure will now be discussed with respect to the characteristics of a photographic material to be processed. In photon-mode exposure, the intrinsic sensitivity (the quantity of energy necessary for the reaction for image formation) of a photographic material is always constant with respect to exposure power density (W/cm2) (=energy density per unit exposure time). In heat-mode exposure, the intrinsic sensitivity increases with an increase in the exposure power density. Now, the exposure time is fixed to be enough for the necessary processability of practicable image-recording materials, and the two modes are compared for the thus-fixed exposure time. In photon-mode exposure, in general, a low degree of energy, about 0.1 mJ/cm2 or so, may be enough for high-sensitivity exposure of the material, but even a slight amount of exposure will cause photo-reaction in the material. Therefore, in this mode, materials often involve a problem of low-exposure fogging in a non-exposed area. On the other hand, in heat-mode exposure, photographic materials do not undergo photo-reaction if the amount of exposure is not above a certain level. In this mode, in general, the photographic material requires a level of exposure energy of 50 mJ/cm2 or so in view of thermal stability, and is therefore free from the problem of low-exposure fogging in the non-exposed area.
In heat-mode exposure, photographic materials require an exposure power density of at least 5,000 W/cm2 on their surface, preferably at least 10,000 W/cm2. Further, although not described in detail herein, high-power density lasers, higher than 5.0xc3x97105 W/cm2, are undesirable as they cause ablation and soil light sources and the like.
Components constituting a photosensitive layer of a planographic printing plate precursor of the present invention are now described.
(C) Onium Salt Having a Counter Anion with a Valency of at Least 2
One characteristic component of the photosensitive layer in the planographic printing plate precursor of the present invention is (C) an onium salt having a counter anion having a valency of at least 2 (this will be hereinafter referred to as a polyvalent anionic onium salt (C)).
A cation site of the polyvalent anionic onium salt structure for use in the present invention may include, for example, those of known diazonium salts, iodonium salts, sulfonium salts, ammonium salts, pyridinium salts and azinium salts. Preferred for the cation site structure of the onium salt are those of sulfonium salts, iodonium salts, diazonium salts, azinium salts and ammonium salts.
Concretely, preferred examples of the onium salt are selected from the group consisting of iodonium salts represented by the following general formula (1), diazonium salts represented by the following general formula (2), and sulfonium salts represented by the following general formula (3). Of these, triarylsulfonium salts and diaryliodonium salts are more preferred in view of safety.
Ar11xe2x80x94I+xe2x80x94Ar12Z11xe2x88x92xe2x80x83xe2x80x83(1) 
Ar21xe2x80x94N+xe2x89xa1NZ21xe2x88x92xe2x80x83xe2x80x83(2) 
In formula (1), Ar11 and Ar12 each independently represent an optionally substituted aryl group having at most 20 carbon atoms. Preferred examples of the substituent, if present, of the aryl group include a halogen atom, a nitro group, an alkyl group having at most 12 carbon atoms, an alkoxy group having at most 12 carbon atoms, and an aryloxy group having at most 12 carbon atoms. Z11xe2x88x92 represents a counter anion having a valency of at least 2, which will be described in detail hereinunder.
In formula (2), Ar21 represents an optionally substituted aryl group having at most 20 carbon atoms. Preferred examples of the substituent for the aryl group include a halogen atom, a nitro group, an alkyl group having at most 12 carbon atoms, an alkoxy group having at most 12 carbon atoms, an aryloxy group having at most 12 carbon atoms, an alkylamino group having at most 12 carbon atoms, a dialkylamino group having at most 12 carbon atoms, an arylamino group having at most 12 carbon atoms, and a diarylamino group having at most 12 carbon atoms. Z21xe2x88x92 has the same meaning as Z11xe2x88x92, representing a counter ion.
In formula (3), R31, R32 and R33 may be the same or different, each representing an optionally substituted hydrocarbon group having at most 20 carbon atoms. Preferably, R31, R32 and R33 are all aryl groups, each of which may be substituted. Preferred examples of the substituent include a halogen atom, a nitro group, an alkyl group having at most 12 carbon atoms, an alkoxy group having at most 12 carbon atoms, and an aryloxy group having at most 12 carbon atoms. Z31xe2x88x92 has the same meaning as Z11xe2x88x92, representing a counter ion.
The anionic structure having a valency of at least 2 of the counter ion in the polyvalent anionic onium salt (C) is not specifically defined, but has at least two anionic sites in one molecule. The at least two anionic sites may be the same or different.
The polyvalent anionic structure is preferably a divalent to hexavalent anion, more preferably a divalent, trivalent or tetravalent anion. Most preferably, it is a divalent anion in view of a synthesis process of the onium salt (C).
Preferably, the anionic site is a conjugated base of a carboxylic acid, a sulfonic acid, a phosphonic acid, a phenol or R1xe2x80x94SO2xe2x80x94NHxe2x80x94R2 (in which R1 and R2 each represent a monovalent, non-metallic organic group). In view of the stability and the reactivity of the onium salt having it, more preferred is a conjugated base of a carboxylic acid, or a conjugated base of a sulfonic acid. Most preferred is a conjugated base of oxalic acid.
Examples of the divalent, trivalent and tetravalent anionic structures preferred for use in the present invention are mentioned below, to which, however, the present invention is not limited. 
The cation sites of the onium salt mentioned hereinabove are applied as counter cations of these divalent, trivalent and tetravalent counter anionic structures. The onium salt may have matching cations, or two or more different types of cations combined. The polyvalent anionic onium salt in the present invention may be a mixture of such an onium salt having matching cations and an onium salt having two or more different types of cations combined.
Examples of the divalent, trivalent or tetravalent anionic structure-having onium salt preferred for use in the present invention are mentioned below, to which, however, the present invention is not limited. Compounds (SA-1) to (SD-8) mentioned below are examples of a sulfonium salt compound having a divalent anionic structure and matching cationic structure; compounds (SE-1) to (SG-6) are examples of a sulfonium salt compound having a divalent anionic structure and different types of cationic structures; compounds (SH-1) to (SH-3) are examples of a sulfonium salt compound having a trivalent anionic structure and matching cationic structure; and compounds (SI-1) and (SI-2) are examples of a sulfonium salt compound having a tetravalent anionic structure and matching cationic structure. 
Compounds (IA-1) to (IF-8) mentioned below are examples of an iodonium salt compound having a divalent anionic structure and matching cationic structure; compounds (IG-1) to (IH-7) are examples of an iodonium salt compound having a divalent anionic structure and different types of cationic structures; compounds (IJ-1) to (IJ-3) are examples of an iodonium salt compound having a trivalent anionic structure; and compounds (IK-1) and (IK-2) are examples of an iodonium salt compound having a tetravalent anionic structure. 
Compounds (ISA-1) to (ISB-6) mentioned below are examples of an onium salt compound having a divalent anionic structure and having sulfonium and iodonium for the cationic structures. 
Preferably, the onium salt for use in the present invention has a maximum absorption wavelength of at most 400 nm, more preferably at most 360 nm. By including an onium salt of this type, having absorption in a UV wavelength range, the image-recording material can be handled even under white lights.
Typical examples of production of the polyvalent anionic onium salt (C) are shown below.